The skin is the largest organ of the body and protects the body from the environmental damage. This protection is provided by the stratum corneum or horny layer of the skin. In this regard, the stratum corneum acts as a barrier (also known as "water barrier" or "permeability barrier") between the body and the outside environment.
It is now generally accepted that the stratum corneum lipids are the key constituents for a functional barrier. Major classes of stratum corneum lipids include cholesterol, free fatty acids, and ceramides. These lipids are synthesized inside the epidermal cells of the skin and are then secreted into the space between these cells, where they assemble into lamellar bilayer sheets to provide a permeability barrier. The stratum corneum serves as a gate keeper that prevents the entry of infection, chemicals, and other pollutants into the skin. In addition, the stratum corneum prevents the loss of moisture from the skin and thus helps maintain a proper intracellular milieu for normal cellular functions. In addition to providing a permeability barrier, skin lipids are important for the maintenance of the skin's shape, form, and healthy youthful appearance. Therefore, the skin lipid, its integrity, amount, and the ability to renew itself are crucial for esthetic appearance, such as decreasing wrinkles and other signs of aging.
During youth, the blood circulation delivers to the skin all the necessary ingredients for lipid synthesis. However, as we age, the blood flow to the skin decreases. This results in decreased delivery of the lipid building nutrients to the skin. The net result is diminished lipid synthesis and decreased lipid contents of the skin of the aging population (J. Clin. Invest. Vol. 95, pp. 2281-2290, 1995). Depletion and inadequate replenishment of skin lipids leads to moisture loss, dryness, skin wrinkles, and altered appearance. Therefore, restoration of skin's lipid contents is crucial for both health and esthetic reasons.
To improve the skin barrier, publications disclose compositions containing natural or synthetic skin lipids. For example, U.S. Pat. No. 5,643,899 discloses the use of lipids for epidermal moisturization and repair of barrier function. However, it is uncertain whether the lipid composition of these products mimic the composition of the human skin lipids. These products contain only from one to three types of lipids, whereas skin lipids are made up of hundreds of types of lipids. In many instances, lipids in skin care products may have been derived from human and/or animal tissues and thus carry the risk of being contaminated with microorganisms such as viruses and/or bacteria. Furthermore, because lipids in general are unstable, the lipids in these products may undergo peroxidation, and the peroxidation products of lipids may cause harm to the skin. Finally, scientific studies have shown that exogenous lipids, including ceramides, actually impede rather than improve the skin's barrier functions. Because of these limitations and concerns about these products, cosmetic compositions which can enhance endogenous production of a correct mix of a full spectrum of physiological lipids by the epidermal cells are highly desirable.
Skin care compositions are known which include some of the compounds disclosed herein. For example, branched-chain amino acids have been employed in skin treatment composition for the treatment of burns, cuts, abrasions, insect bite, dry skin, psoriasis, dermatitis, eczema, and inflammation (U.S. Pat. No. 5,425,954). Sarpotdar, U.S. Pat. No. 4,732,892 discloses a composition for transdermal penetration enhancers containing branched-chain amino acids. Ciavatt, U.S. Pat. No. 4,201,235 discloses a composition for skin, hair, and scalp conditioners containing several amino acids including the branched-chain amino acids. Morelle, U.S. Pat. No. 4,859,653 discloses the use of derivatives of branched-chain amino acids (butyrylvaline and butyrylleucine) for use in treating wrinkling of the human skin.
The role of branched-chain acyl coenzyme A (CoA) to produce fatty acids in the skin was postulated more than 20 years ago (Nicolaides: Science, 186: 19-26, 1974). However, only recently the incorporation of carbon skeletons of branched-chain amino acids into skin lipids of laboratory animals has been demonstrated (Oku et. al.: Biochim. Biophys. Acta 1214: 279-287, 1994).
The art also discloses other compounds individually used in skin care. For example, U.S. Pat. No. 5,472,698 discloses a composition containing lipid building ingredients (serine or its derivatives). However, these ingredients are capable of producing a single class of skin lipids, namely ceramides, and do not include components to produce a full spectrum of skin lipids, namely cholesterol, free fatty acids, and ceramides.
Similarly, skin care compositions are also known to include caprylic acid (also known as octanoate or octanoic acid), either as free acid, but more often in an esterified form as caprylic/capric acid triglycerides. For example, U.S. Pat. No. 5,175,190 discloses a composition for the treatment of skin lesions containing caprylic/capric triglycerides. U.S. Pat. No. 5,569,461 discloses a topical antimicrobial composition containing a monoester of caprylic acid. U.S. Pat. No. 4,760,096 discloses a moisturizing skin preparation containing caprylic/capric acid triglycerides. U.S. Pat. No. 4,495,079 discloses a composition for facial skin cleanser capable of softening and removing sebum plaque containing a mixture of caprylic acid and capric acid esterified to a fatty alcohol. U.S. Pat. No. 5,472,698 discloses the use of several thiol compounds, including the use of lipoic acid in enhancing lipid production in the skin.
There remains a need, however, for compositions and methods that among other things increase lipid production in the skin.